What is except* and ExceptionGroup in Python 3.11+?

Q: What is except* and ExceptionGroup in Python 3.11+?

**Target Audience:** Python developers who know try/except and are meeting exception groups for the first time **Difficulty:** Medium **Python version:** `ExceptionGroup` and `except*` need Python 3.11+; checked on CPython 3.11 (and in this page on Pyodide 0.25.1, which is CPython 3.11.3), the asyncio example on CPython 3.12 > **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. Every output block in the **Verified** section was copied from a real run of the examples on this page. ## 1. Why This Even Matters — A Story First You send three friends to three shops for party supplies. One comes back saying the bakery was closed. Another says the drinks shop had no ice. The third got everything. If each friend could only report "something went wrong", you would hear one problem and never learn about the other. What you actually want is a single report with **both** problems

The problem

Question presented to candidate: "Python 3.11 added ExceptionGroup and a new except* syntax. What problem do they solve, and how does except* behave differently from a normal except when a group contains several different errors?"

What a strong answer should cover:

The problem: concurrent tasks or bulk validation can fail in several unrelated ways at once, and a normal exception can only report one; ExceptionGroup(message, [exc, ...]) carries them all (PEP 654, Python 3.11)
except* T matches the leaves of the group by type, at any nesting depth, and binds a new group holding only the matching leaves; several except* clauses can run for one group
Leaves that no clause matches are re-raised as a group, preserving the nesting; a plain exception caught by except* arrives wrapped in a group
Rules: you cannot mix except and except* on one try; break, continue and return are not allowed inside except*; except* ExceptionGroup is a TypeError
ExceptionGroup is an Exception; BaseExceptionGroup is used when a leaf is a BaseException such as KeyboardInterrupt
Where groups come from in practice: asyncio.TaskGroup (3.11), and the exceptiongroup backport for older Pythons

Code / implementation expected: Yes — raise a group of two or three different errors and handle them with separate except* clauses, showing what each handler receives.

exceptions
except-star
python3.11

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Solution

Target Audience: Python developers who know try/except and are meeting exception groups for the first time Difficulty: Medium Python version: ExceptionGroup and except* need Python 3.11+; checked on CPython 3.11 (and in this page on Pyodide 0.25.1, which is CPython 3.11.3), the asyncio example on CPython 3.12

How to read this doc: Each idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact word an interviewer expects. Every output block in the Verified section was copied from a real run of the examples on this page.

1. Why This Even Matters — A Story First

You send three friends to three shops for party supplies. One comes back saying the bakery was closed. Another says the drinks shop had no ice. The third got everything. If each friend could only report "something went wrong", you would hear one probl

The solution is hidden so you can try first

12 min read · 10 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

PythonRaising a group and splitting it with except*

Practice

PythonThe rules: no mixing, no return, first match per leaf, errors raised in a handler

Practice

Pythonasyncio.TaskGroup raising an ExceptionGroup (needs a real event loop)

import asyncio

# Real-world source of groups: asyncio.TaskGroup (3.11+).
async def fetch(name, delay, fail_with=None):
    await asyncio.sleep(delay)
    if fail_with:
        raise fail_with
    return name + " ok"

async def main():
    try:
        async with asyncio.TaskGroup() as tg:
            t1 = tg.create_task(fetch("users", 0.01))
            tg.create_task(fetch("orders", 0.02, ValueError("orders: bad JSON")))
            tg.create_task(fetch("stock", 0.02, ConnectionError("stock: refused")))
        print("not reached")
    except* ValueError as eg:
        print("parse errors:", [str(e) for e in eg.exceptions])
    except* ConnectionError as eg:
        print("network errors:", [str(e) for e in eg.exceptions])
    print("users task finished first:", t1.result())

asyncio.run(main())

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